Many hydrocarbon-producing reservoirs (e.g. oil and gas) lie in inhomogenous formations which, in turn, are comprised of different zones having varying permeabilities. Before carrying certain completion and/or production operations in such inhomogenious reservoirs, it is often desirable to treat the reservoir to alter the flow profile of fluids therethrough by blocking or restricting flow through the more permeable zones of a formation while encouraging flow through the less permeable zones.
For example, in order to carry out a successful hydraulic fracturing operation in such inhomogenious formations, it may be necessary or at least desirable to first block off the more permeable zones so that the bulk of the fracturing fluid will not merely flow into these zones and be lost. Further, during primary production from such formations, the oil and/or gas normally flows more readily from the higher permeablility zones than it does from the less permeable zones thereby making the oil lying in the less permeable zone difficult to recover with primary recovery operations.
"Secondary recovery" operations are well known for producing oil and/or gas from a formation. Such operations can be used for the initial or "primary" production from a particular reservoir or they can be used in recovering residual oil that may remain in a formation after other production techniques are no longer ecomonical. In a typical secondary recovery operation, a drive fluid (e.g. water and/or gas) is injected into the formation through an injection well to displace the oil towards a production well.
In inhomogenious reservoirs such as described above, the higher-permeable zones or "thief zones", as they are sometimes called, effectively act as pipelines or conduits between the injection and production wells. If the formation is untreated before the drive fluid is injected, it will almost certainly flow through these more-permeable zones thereby effectively bypassing the less permeable zones. This results in early "break-through" of the drive fluid at the production well(s) which , in turn, results in poor sweep efficiency through the formation and low recovery of the residual oil from the less permeable zones.
Several techniques have been proposed for altering or modifying the flow profile of a reservoir in order to direct the bulk of the injected fluid away from the more permeable zones and into the less permeable zones of the reservoir. One such technique involves injecting a "gelant", i.e. aqueous, gel-forming polymer, into the formation through an injection well. The polymer gel solution will normally take the path of least resistance and flow into the more permeable zones of the producing formation. The gelant is then allowed to "set up" to form a flow-blocking gel in the high-permeable zones. Then, when a fracturing fluid or a drive fluid, e.g. water, is injected into the treated formation, it will bypass the high-permeable zones and be diverted into the less-permeable zones. This results in a more efficient fracturing operation or a better recovery of residual oil depending on the operation involved.
Specific examples of gelants, i.e. polymer gels, which have been proposed for treating subterranean formations are set out and discussed in the following references: "Impact of Permeability and Lithology on Gel Performance", R. S. Seright et al, (SPE/DOE 24190); "Gel Placement in Heterogeneous Systems with Crossflow", K. S. Sorbie, (SPE/DOE 24192); and "A New Gelation Technology for In-Depth Placement of Cr.sup.+3 /Polymer Gels in High-Temperature Reservoirs", T. P. Lockhart et al, (SPE/DOE 24194); all of which were presented at the SPE/DOE Eighth Symposium of Enhanced Oil Recovery, Tulsa, Okla., Apr. 22-24, 1992.
While polymer gel treatments such as described above have been tested in the laboratory and have shown promise in the field, the gelants, themselves, (e.g. polyacrylamides) are relatively complex and expensive to use in most applications.